Southern California’s wettest December in nearly a decade quashed any danger lingering from destructive wildfires in fall, but experts warn that red flag conditions could return as early as April.
The series of storms that drenched the state the past few months appear to be part of a trend toward weather extremes. They came on the heels of autumn’s record-high temperatures and furious, wind-driven fires.
The abrupt change came when a high-pressure system above the eastern Pacific Ocean slipped slightly west at the end of November after having kept the state dry since spring, according to the latest Predictive Services wildfire outlook from the Southern California Geographic Coordination Center in Riverside.
The pressure system appears to be tied to high sea surface temperatures. It essentially blocks jet streams from bringing wet weather into California, said Robert Krohn, a fire weather meteorologist with Predictive Services.
“I think it’s going to have long-term effects in terms of making us dryer but perhaps more extreme,” Krohn said, cautioning that such systems are complex. “The weather patterns that were more often and typically seen are not really present in some years.”
The same high-pressure system is blamed for obstructing fall storms the previous two years, as well. Fall of 2017 and 2018 saw deadly, record-breaking wildfires in the state.
California needed the dousing it got the past few months. But, ideally, rain would come steadily rather than in a few intense storms, says Casey Oswant, a meteorologist at the National Weather Service’s San Diego office.
“When you get extreme totals like that it tends to cause problems with flooding, especially in the Orange County and Los Angeles areas because there are a lot of structures and pavement that prevent water from soaking into the ground,” she said.
The Pacific pressure system will soon move back east, however, and because of that forecasters expect this winter to be drier than normal.
January and February should see the greatest rainfall deficits.
SoCal fall expected to become hotter and drier, creating danger as soon as winds kick up
Before the pressure system deviated west in November, Southern California had an unusually warm and “exceptionally dry” autumn, says Mark Jackson, a meteorologist at the National Weather Service’s Oxnard office.
“In September and October, sometimes we get thunderstorms,” Jackson said, “and we didn’t get anything like that this year.”
The Southern California rainy season typically begins around the middle of October, but hot and dry conditions were widespread at that time in 2019. Fanned by Santa Ana winds, a rapid and dramatic succession of blazes in October charred about 187 square miles statewide, roughly the size of Palmdale and Lancaster combined.
At the end of that month, 11 wildfires were burning simultaneously across Southern California.
October’s red flag conditions were “unprecedented,” Jackson said.
“What made this October so unusual was how close each one of those events were,” he said. “That was unusual, since I’ve been working here, to have them so close together — not only on a weekly basis but on a biweekly basis. It’s kind of crazy. We had ongoing fires from previous Santa Anas, and then a new Santa Ana comes up.”
Under similarly dry conditions in 2017, the Thomas Fire became the largest blaze in state history and the Tubbs Fire the most destructive, before those records were broken by the Mendocino Complex and Camp fires. In 2018, flames consumed a chunk of land larger than Ventura County and set a new record for the total area burned statewide.
Late-season fires are expected to continue as Southern California — and the globe — grow warmer. The last major wildfire of 2019, the Cave Fire north of Santa Barbara, broke out Nov. 25, just before the season’s first major storm.
The amount of land burned across the state annually has quintupled since 1972, according to a study published in July in Earth’s Future.
That’s largely because fall rain is coming later, leaving vegetation extremely dry and ready to spark when strong winds arrive in November, the researchers found.
They expect large autumn wildfires to become more frequent across the state.
The state’s been moving toward this new reality for some time. Cal Fire stopped using the term “fire season” nearly 15 years ago, and the phrase has “kind of been retired,” Krohn says.
“It’s not like other states, where summer is the fire season,” he said. “It just depends upon what part of the state you’re talking about, and in some parts of the state it lasts all year.”
In 2020, fire danger could return to SoCal by early April
Widespread 90-degree temperatures persisted past the middle of November, and the start of 2020 is also expected to be warmer than normal.
“Pretty soon, we’re going to call this normal,” Jackson said.
Precipitation is forecast to be lightest during the rainy season’s normal peak, in January and February, which Krohn says means vital snowpack could drop low and drought conditions may return.
Paired with high temperatures, that would allow hillsides to dry out earlier than usual, and warm, windy weather could bring elevated fire danger in the first weeks of April.
“A lot of times it gets going in late April or May, and that might get advanced by a month or so,” Krohn said. “We’ve seen that happen where we get quite a bit of rain in the middle of winter, and then we dry out in the spring.”
Red flag conditions will arrive first in the Southern California foothills and interior Central Coast, which won’t be protected by snowpack and will dry out first when strong onshore winds return. Krohn noted that both areas also have brush ripe for burning at higher elevations.
Vegetative health plays just as important a role as climate in sparking wildfires, Krohn said, and Southern California’s is a problem.
“It’s so poor in a lot of areas that the damage has been done,” he said. “In foothill communities and at higher elevations, there’s millions of dead trees. That’s going to be a problem for many years, especially once we get to the hotter months. The combination is not a real winning one.”